Well you can turn on java2d trace logging to see what calls Java2D is doing under water (google it).

But likely you can't do anything. Java2D performance is really unpredictable and also partially bound to the quality of your video drivers and whatever else the computer is doing. On top of that Java2D development has basically been halted in favor of the JavaFX 2 prism rendering engine. You could try disabling Direct3D hardware acceleration to see if that takes the problem away; in my experience the old software rendering pipeline used prior to Java 6 update 10 was more stable.